Lithospheric structure of the central Himalaya from 3-D tomographic imaging

The High Himalaya are one of the most spectacular consequences of the ongoing collision between India and Eurasia, yet the role that the lithosphere beneath them play in this collision is not well understood. To illuminate the influence of the Himalaya on collisional processes, we produced tomographic images of the subsurface beneath them from arrival times of P- and S-waves generated by 551 teleseisms and 1051 local/regional events and recorded by the Himalayan–Nepal–Tibet Broadband Seismic Experiment (HIMNT) from 2001 to 2002. Compared to those in initial 1D models, the RMS P- and S-arrival time misfits in our final 3D model were reduced by as much as 46% (from 0.45 s to 0.24 s) and 38% (from 0.97 s to 0.6 s), respectively. Our results show strong crustal variations perpendicular to, but no significant lateral variation along, the trend of the High Himalaya within our study area. In the NS direction the crust thickens from about 60 km under the Lesser Himalaya to 70–80 km under the Tethys Himalaya. There is no localized crustal thickening beneath the High Himalaya. Wavespeeds and seismicity patterns beneath the Tethys Himalaya suggest that the middle crust (at 30–50 km depth) is ductile and are consistent with the presence of a mid-crustal reflector associated with the stronger gradients in the lower crust. Correlation between relocated earthquakes and the Xainza-Dinggye rift indicates that the rift is not cutting through the crust at the present time. Shear wave splitting from core phases indicates that the upper mantle beneath the Tethys Himalaya is nearly isotropic. The combination of isotropy and higher seismic wavespeed leads us to suggest that eclogites included in the lowermost crust and the uppermost mantle are responsible for the mid-crustal reflector under the Tethys Himalaya, as well as for a slightly high wavespeed anomaly just beneath the Moho. This interpretation is supported by the existence of granulitic eclogite in the Ama Drime Massif ∼ 50 km northeast of Mt. Everest.